The purpose of the present invention is to provide an isoprene oligomer and polyisoprene in which a modification has been added to the main-chain moiety of the molecule. Also provided are a rubber composition obtained by blending the isoprene oligomer and/or polyisoprene, and a pneumatic tire in which the rubber composition is used for the members of the tire (for example, the tread or sidewalls). The present invention pertains to an isoprene oligomer obtained by being synthesized from an allylic diphosphoric acid represented by formula (X), and a compound represented by formula (Y). (In formula (X), n represents an integer 1 to 10. In formula (Y), R represents a group other than a methyl group.)